The invention relates to a fiber-optic gyro, and in particular a fiber optic gyro of the Sagnac type.
Fiber-optic gyros of this generic type are generally known. However, a problem which is encountered with respect to such prior art devices is that changes of the characteristics of the individual components of the fiber-optic gyro, (for example, the light source or the 3.times.3-coupler constructed of three optically coupled fiber-optic light guides) cause errors in the measurements.
German Patent Document DE 38 05 904 A1 discloses an arrangement to detect changes of the characteristics of the fiber-optic gyro and to make corresponding corrections, by means of a light detector provided on the exit side of the input light guide, and a second detector which monitors the intensity of the light emitted by the light source. Although, this known fiber-optic gyro can detect and eliminate control-caused changes of the light source as well as various changes of the 3.times.3-coupler, systemic measurement errors continue to occur due to uncorrected changes of the coupling values between the individual fiber-optic light guides.
German Patent Document P 39 12 005.8 discloses a process for signal analysis of a fiber-optic gyro having a 3.times.3-coupler which provides an improved determination of the rate of rotation despite stochastic fluctuations in contrast of the interference signal to be analyzed. Nevertheless, zero-point drifts and scale factor drifts remain problematic with this fiber-optic gyro as a result of environmentally caused fluctuations of the 3.times.3-coupler.
It is therefore an object of the present invention to provide an improved Sagnac-type fiber-optic gyro in which the influence of environmental conditions on the 3.times.3-coupler causes no zero-point drifts and no scale-factor changes.
This object and other objects and advantages are achieved according to the invention, in which a control and analysis unit controls the intensity and the center wave length of the light emitted from the light source in such a manner that the ratio of the output signal of the light detector (and the detector provided on the exit side of the input light guide) to the output signal of the monitoring detector (the detector which detects the intensity of the light emitted by the light source) remains constant, without any interposition of a coupling light guide.
The invention is based on the principal consideration that the three coupling values between the individual ducts of the 3.times.3 coupler are given by the product of the length of the joint coupling path and the respective coupling constant k.sub.ij. The coupling constants in turn depend essentially on the distances of the respective light guides from one another, on the wave length of the coupled-in light and on environmental conditions (pressure, temperature, etc.). The individual coupling constants, as recognized according to the invention, have essentially uniform dependency on the environmental conditions of pressure, temperature, etc. as well as the wave length. As a result, as also recognized according to the invention, it is possible to compensate for environmentally caused changes of the coupling constants by a corresponding change of the center wave length of the coupled-in light if, in response to changes in the intensity of the light source, the pertaining center wave length is varied in such a manner that the ratio of the output signal of the light detector to the output signal of the monitoring detector is constant.
For the analysis of the output signals of the two measuring detectors which are mounted on the ends of the additional light guides opposite the fiber-optic coil, it is particularly advantageous for the control and analysis unit to control the intensity and the center wave length of the light emitted by the light source in such a manner that not only is the ratio of the two output signals constant, but also the output signals of the light detector and of the monitoring detector themselves are constant as well. Thus their ratio is naturally also constant.
The variation of the center wave length of the light source may be achieved in a variety of ways. In each instance, however, it is advantageous in view of the overall size, sensitivity, etc. for the light source to be a semiconductor light source, such as an LED, ELED or a semiconductor laser, for which variation of the emitted light may be achieved either by manipulation of the temperature of the semiconductor light source or by variation of the injection current. The temperature of the semiconductor light source may be varied by a tempering element which may be any arbitrary heating and/or cooling device. Especially advantageous for this purpose is a Peltier element, in which heat is generated or absorbed at the junction of two dissimilar metals which carry a small current
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.